Robots can generally be described as being machines that have a shape similar to a human, act like a human, or have a self-working capability. Robots that have the capability of perceiving an environment and recognizing the situation resulting from such an environment are referred to as intelligent robots. Robots that have a shape similar to that of human or have movements that resemble a human, are referred to androids or humanoids.
Research and development on robots have been made in various fields for toys, pets, services, security, extremely dangerous environmental work, advertisement, etc. Such research and development focus upon more complicated mechanisms, e.g., artificial intelligence, and shows various facial expressions in addition to biped working function, to create robots that more closely resemble a human.
Robots for toys, pets, advertisement, etc, in particular an android or humanoid, needs to show diverse emotional facial expressions. Such facial expressions can be performed by changing positions of eyebrows, eyeballs or changing the shape of lips. Further, lip-synching can be performed by appropriately changing the shape of lips.
With regard to technologies for changing lip-shapes to show change in facial expressions and perform lip-synching change of the facial expressions, existing technologies use an indirect lip moving method, wherein driving forces are applied to a predetermined point of an artificial skin or muscle adjacent to the lip member, and a direct lip moving method, wherein lips are directly moved.
With Reference to FIGS. 1 to 2, a prior art method for moving robot lips and a prior art lip moving device are described.
FIG. 1 illustrates various facial expressions that can be shown by a prior art robot. FIG. 2 illustrates various example movements of lips shown in FIG. 1. FIG. 1 illustrates facial expressions, which a robot ‘Icat’ manufactured by Philips Co., Ltd. can show. It can show five facial expressions such as expressionlessness, curiousness, smile, anger and laugh by changing positions of lips 10, an eyeball 20 and an eyebrow 30. Torque is applied to ends of an upper lip 10a or a lower lip 10b to change the position of the lips 10.
The prior art lip moving device can rotate both ends of the upper lip 10a and the lower lip 10b respectively. As shown in FIG. 2, the prior art lip moving device has a first motor 11a fixed to one end of the upper lip 10a, a second motor 12a fixed to the other end of the upper lip 10a, a third motor 11b fixed to one end of the lower lip 10b and a fourth motor 12b fixed to the other end of the lower lip member 10b. 
To show a facial expression such as expressionlessness with lips closed, as shown in FIG. 2(a), the first to fourth motors 11a, 12a, 11b, 12b do not operate and the upper lip 10a and the lower lip 10b are maintained in a horizontal state.
To show a facial expression such as curiousness, 10b as shown in FIG. 2(b), the first to fourth motors 11a, 12a, 11b, and 12b counterclockwise rotate such that the upper lip 10a and the lower lip 10b form a waved shape as the upper lip 10a contacts the lower lip.
To show a facial expression such as smile, as shown in FIG. 2(c), the first and third motors 11a, 11b clockwise rotate and the second and fourth motors 12a, 12b counterclockwise rotate, thereby downwardly curving the upper and lower lips 10a, 10b as the upper lip 10a contacts the lower lip 10b. 
To show a facial expression such as anger, as shown in FIG. 2(d), the first and third motors 11a, 11b counterclockwise rotate and the second and fourth motors 12a, 12b clockwise rotate, thereby upwardly curving the upper and lower lips 10a, 10b as the upper lip 10a contacts the lower lip 10b. 
To show a facial expression such as a laugh, as shown in FIG. 2(e), the first and fourth motors 11a, 12b counterclockwise rotate and the second and third motors 12a, 11b clockwise rotate, thereby upwardly curving the upper lip 10a and downwardly curving the lower lip 10b, as a middle section of the upper lip 10a is separated from a middle section of the lower lip 10b. 
However, the facial expressions, which the prior art lip moving device can express, are limited as explained above. Thus, the prior art lip moving device fails to show various facial expressions and to perform lip-sync through delicate lip shape change.